Process of coating metal and resulting articles



Nov. 3, 1959 L. K. SCHUSTER ETAL 2,9

PROCESS OF COATING METAL AND RESULTING ARTICLES Filed May 25, 1956 wow/a K. SCHUSTER a ALFONSO L. 3.41.01 JR.

mmvrons.

THE IR ATTORNEYS United States Patent PROCESS OF COATING METAL AND RESULTING ARTICLES Ludwig K. Schuster, Philadelphia, and Alfonso L. Baldi, Jr., Drexel Hill, Pa., assignors, bymesne assignments, to Kelsey-Hayes Company Application May 25, 19s6,-sei-ia1 No. 587,388

13.Claims. (c1.- 14896.2)

This invention relates to the coating of metals, for the purpose of improving their corrosion; resistance as well as the adhesion of organic top coatmg's such as .paints, lacquers, or resins in general.

' being mixed together only just before spraying.

reference being made to the accompanying drawingin which Fig. 1 shows an aluminum wire coated in accordance with the present invention, and Fig. 2 is a partially broken away view of a container made from coated metal and illustrative of the present invention.

According to the present invention, a metal face is coated with an aqueous solution essentially of chromic acid and a compatible reducing agentfor the chromic acid, and the coated surface is heated to a temperature of from 250 to 500 degrees F. to cause the reducing agent to react with the chromic acid and leave a waterinsoluble layer, the proportions of reducing agent and chrom-ic acid being such, that from about 40 to about 95% of the chromium'inthechromic acid is reduced to trivalent form, and the final layer weighs at least about 3 milligrams per squarefoot of surface. With metals such as zinc, cadmium, magnesium and aluminum, considerably better results areobtained when the coating weight is above 10 milligrams per square foot.

The reducing agent is preferably one that does not leave water-soluble salts in the final coating layer.' Compatible reducing agents are those that when present in the above proportions do not cause any precipitation during the period in which the solution' is kept as a solution,

and are not driven off by the heat treatment before they are oxidized. Organic polyalcohols are very efifective reducing agents, and examples of specifically desirable polyalcohols include sucrose, invert, su'gar,:-dextrose, glycols, polyglycols such as diethyle'ne glycol, glycerine, manni-tol, sorbitol, triethanolamine and tartaric acid.

The reducing agents listed above do not react very rapidly and can be stored in a chromic acid solution for at least one day at room temperature (80 F.) without deterioration. 1

In general, polyalcohols and their oxidation products are suitable reducing agents. These contain a carbon atom to which is linked an alcohol group that is not protected against oxidation, and such structure appears to be effective regardless of the remainder of the molecule, so long as the materials are not too volatile.

l The presence of volatilizable material in. the coating bath in general does not detract from its efiicacy, if it does not interfere with the reducing operation. It fact, some additives such as ammonia appear to improve the corrosion resistance and lacquer-adhesion of the coating on tin-plated steel for instance. The addition of ammonia can for example be made to the point where the CrO -reducing agent bath is rendered slightly alkaline (pH 7.4 for example), although greater or lesser additions of ammonia are also helpful. A suitable pH range is from 3 to 10. The addition ofammonia is most significant when coatings weighing less than 10 milligrams foot are applied.

The concentrationof the components in the coating solution will depend upon the weight or thickness of the coating desired-the heavier coating requiring the use of the more concentra'ted'solutions. The ratio of the chromic acid to the reducing agent will vary, depending upon the reducing agent selected, but such a ratio should beused that 40 to 95% of the total chromium is reduced, to'

Other reducing agents suchas citric acid and hydroxylamine salts, such as hydroxylamine sulfate and even phosphorous acid are also suitable. Although reducing agents such as potassium iodide and phenol can bef used, they produce coatings that are not too satisfactory. The reducing agent should not be so stable thatit cannot be oxidized by the chromic acid, at least when it reaches the final baking temperature. A coating bath containing both the chromic acid and the reducing agent should not be permitted to stand long enough to form deposits of insoluble chromium compounds before the bath is applied to the surface and cured,, that is, subjected to the final heat treatment. To avoid this, the dwell of the reducing agent in contact with the chromic acid solution should be of limited duration. The chromic acid and the reducing agent can even be applied as by spraying from a com ing solution with sodium thiosulfate, and" subtracting this from the total chromium content which is determined by oxidizing another dissolved coating sample with H 0 and again titrating with sodium thiosulfate.

The greater the trivalent chromium content, the more insoluble the coating and the more resistant it becomes to chemical attack. However, the greater resistance to attack requires longer treatment for dissolution of the coating in the above analytical procedure. When the coating is applied over zinc, aluminum or other metal soluble in caustic alkali, the hexavalent chromium content is reduced during the dissolving step by the chemical action on the metal. Apparently the nascent hydrogen generated by the action effects the reduction. The hexavalent chromium content that any coating bath'provides on such reactive metals, can be determined by applying the coating bath in exactly the same way to an alkaliresistant metal, or to surfaces like glass which do not interfere with the analysis, since glass even if attacked by alkali does not appear to cause any change in the hexavalent chromium content. Coatings on glass can be dissolved in more concentrated alkali, such as those containing 40% NaOH by weight. The coating formed on glass appears to be identical'to that formed on metal.

The aqueous chromium-oXide-depositing film applied to the metal surface in accordance with this invention is converted to a water-insoluble solid when it is subjected to a temperature at least as high as 250 F. Only a fraction of a second is required for the conversion after the converting temperature is reached. However, some time may be required to reach that temperature, depending upon the rate at which heat is supplied, the thickness and mass of metal, and the heating technique. Anysource of heat is applicable. The selection depends upon the per square some of the reducing agent.

desired .conversion speeds, article belng heated, etc. Suitable heating sources are, for instance, infra red lamps, cup-type ceramic or tubular metal gas burners heated to incandescence, flames as with ordinary gasburners, induction heating, hot air and the like.

Somewhat better coatings are also obtained if the coating is cured at the higher temperatures and with some metals like zinc, if the curing treatment is prolonged as for example to .5 or 10 seconds or even longer. However, since commercial requirements are for high speed treatments (for instance 600 to 1000 feet per minute), it is most practical to apply the mixed chromium oxide While the metal is moving at that speed. For this purpose the" conversion can be arranged to take place almost instantaneously by using heat sources at higher temperatures which brings the aqueous film rapidly to 400 F., for example. Longer cures are effected at these high speeds by using longer curing ovens or burners.

The coating solution can be applied by immersion, flooding, spraying, roller coating, etc. Roller coating or spraying is particularly suitable for continuous coating of tin plated strip. The thickness of the coating depends upon the concentration of the solution. If applied by roller coating, the roll pressure and roll surface characteristics also affect the thickness of the final coating.

For commercial applications it is desirable to continually monitor the chromium oxide coating, as for example to make sure it has the desired thickness or weight. Over a magnetic base, such as on tin-plated plain carbon steel, the thickness of the coating, which is non-magnetic, inserts a correspondingly varying reluctance in a magnetic circuit the entire treatment time is held to a minimum. The

final coating is also more uniform and precleaning probl'ems are minimized. Furthermore, oxidizable wetting agents such as are formed by condensing 3 mols of ethylene oxide with p-(n-octyl) phenol, as described in U.S. PatentNo. 2,115,192, may be used to take the place of Long-chained alkyl sulfates are suitable and tertiary butyl alcohol will be effective although it does not cause any appreciable reduction and is generally required to be in higher concentration, e.g. 0.5%, to be really effective, In general, however, anionic, cationic or non-ionic wetting agents including quaternary ammonium compounds of long-chain alkyl or aralkylamines, can be used.

Another feature of the present invention is that the coating does not interfere withwelding. No localized removal of the coating is necessary and resistance welding can, for example, be conducted in'the normal way' used for uncoated metal.

The coating of the present invention can be applied to the surface of any metal to improve the corrosion re- Sistance and paint or lacquer adhesion. The improvements are particularly striking, however, with non-ferrous surfaces such as aluminum and zinc. Other metals that are highly suited for coating include cadimum, copper, silver, magnesium, beryllium, titanium, tin, lead, bismuth, tungsten, cobalt, nickel and alloys of these metals with each other. With tin surfaces the coating of the present invention is preferably used with coating weights less than about.l milligrams per square foot to provide the best adhesion of organic finishes, such as paints, enamels. or lacquers. Tin platings on other 916mb u a P in the shape and thickness of the i carbon steel, are greatly improved by the coating of the present invention.

A feature of the present invention is that the coating is particularly highly resistant to alkaline attack. Metals like aluminum are in some uses quite vulnerable to such attack, as for example, when used in the form of conductor wire of telephone and power supply circuits.

Fig. 1 shows such a wire 6 made of 99.99% aluminum with the coating 8 of the present invention covering it. Such a wire can be laid in underground cables and exposed to the air or the weather without any detectable attack on the aluminum for extremely long periods of time.

Fig. 2 shows a can 10 representing another embodiment of the present invention. The can has a tubular body 12 SAE 1010 steel 31, 10 mils thick plated with a layer of tin 32, that is covered with a coating 34 according to the present invention. The ends of thestrip are joined together as indicated at 13. These ends are folded back on each other to provide interlocking flanges 16 and 18 between which a yielding gasketing layer 17 such as rubher is applied. The flange and gasket combination is then crimped together to make a suitably sturdy and leakproof joint.

The openings at the top and bottom of the tubular body are sealed with covers 20, 22 that have beaded peripheries 24 with interlocking flanges 26, 28. The inter locking beaded periphery can merely be crimped in place, preferably with a layer 30 of gasketing to assure hermetical seal- The following specific examples show some of the ways in which the present invention can be practiced.

Example -I oxide with p-(n-octyl). phenol as described in U.S. Patent No. 2,115,192, the solution being held at 70 F.

Roll the coated metal through rubber rollers wetted with the coating solution. 1 u

Cure in an air furnace held at 1700" F. by passing the rolled'sheet through with a dwell of 15 seconds in the furnace so that the peak temperature of the aluminum surface reaches 400 to 450 F.

The adhesion of organic coatings such as paint to the resulting surface is extremely good and the unpainted coated aluminum sheets show almost no corrosion after standing out in the weather for six months. The paint or lacquer adhesion is even further increased by mechanically or chemically roughening the aluminum surface before the mixed chromium oxide layer is applied. Wire brushing, sand blasting, slurry blasting, or chemically etching will provide an excellent paint bonding surface even though the mixed chromium oxide layer intervenes. An effective chemical etching operation is one carried out in an aqueous solution of, 5 ounces per gallon of sodium hydroxide and 0.1% of a wetting agent such as isopropylnaphthalene sodium sulfonate or the Wetting agent described in Example I. Removal of about 300 to 4000 milligrams per square foot of aluminum gives very satisfactory results.

Example. 11

A 24 mil thick round wireof 38 aluminum was passed in a continuous, manner through the following treatment condition;-

(,1) Three, seconds in an etching, bath of 10% NaOH and 3% sodium gluconate in water held. at F.

' ping plain carbon steels with zinc.

(2) One second rinse-in tap water at 70 F.

(4) One second rinse in tap water at 70 F.

(5) One second inan'aqueous solution of 8% by weight and 2.7% sucrose by weight. I

(6) Three seconds cure in an air oven having an internal temperature of 1200 F., the wire surface reaching 400' F.

The resulting coated wire has a coating of 85 milligrams per square foot and an unusually high resistance to attack by alkali, showing a ten-fold increase in resistance to 0.1 N NaOH as compared to untreated wire. The alkali resistance of the mixed chromium oxide coatings is best when the coatings are cured at temperatures of 400 F. or higher and when the coating weights are above about 20 milligrams per square foot.

As shown in prior application Serial No. 458,668, filed September 27, 1954, and now abandoned, the coatings of the present invention are also. helpful in covering zinc surfaces such as those made by electroplating or hot dip- Without this protection the zinc rapidly develops whiterust, particularly on standing in a humid atmosphere. The disclosures of the prior application is hereby incorporated herein as though fully set forth.

The adhesion of some paints, enamels and lacquers to the mixed chromium oxide coating on zinc is markedly increased when the curing temperature is raised to between 400 to 500 F. The popular melamine-modified alkyd paints or unmodified alkyd paints described in Paint and Varnish Technology by Von Fischer, published 1948, chapter 9, pages 127-144, for example, adhere unusually well to a mixed chromium oxide coating which has'been heated above 400 F., even though the paint itself is cured at 350 F. or below. However, the adhesion of this paint is not as good as phenolic, oil-modified alkyd, or oleoresinous paints when the mixed chromium oxide coating, either before or after the application .of the paint, is not heated as high as 400 F. Paint-covered coatings do not have the corrosion problems of zinc filmed with uncovered mixed chromium oxides, and when a paint is to beapplied there is no significant advantage in having the mixed chromium oxide layer cured for more than one or two seconds.

The adhesion of the mixed chromium oxides to the zinc is so great that it can be used to improve the adhesion of the oxide coating on iron or steel. Thus, a flash plating of zinc weighing only about 0.1 to 0.5 pound per base box, can be deposited as from a potassium zinc cyanide bath on plain carbon steel such as SAE 1010, and provides a surface to which mixed chromium oxide coatings of 5 to 200 milligrams per square foot adhere very tenaciously. The resulting metal is highly suited to fabricating operations as in the manufacture of hermetically sealed cans, with or without an additional paint, enamel or lacquer coating. The paint, enamel or lacquer adheres very well to the oxide coating, so that the wellknown paint-bonding ditficulties of zinc surfaces are also avoided. Cans made in this way are highly suited for holding detergents, paints, etc.

Inasmuch as zinc surfaces are not too easily wetted, as for example when the liquid film that deposits the mixed chromium oxides is applied, it is preferable to always use wetting agents as indicated above, and even to bring the wetting agent concentration to 0.02% or even higher. Furthermore, thejuse of reducing agents like triethanolamine helps improve the Wetting action and is preferred with or without the addition of conventional wetting agents.

. By way of example, one highly effective coating bath for, ,zinc electroplated from either potassium zinc cyanidev I CI'Og (3) Two seconds in 5% HNO by weight water at or acid zinc sulfate baths, or eve'n'd ip-galvanized surfaces is a water solution of the following by weight:

- mags 111 2% C10 7 I 0.6% triethanolamine ene oxide with p-(n-octyl) phenol as described in Patent No. 2,115,192

No appreciable contact time is needed and the conversion can be effected at 400-500 F. for one second when a paint, enamel, lacquer, etc., is to be applied over it.

This treatment used with 0.014 inch thick cold rolled "SAE 1010 steel, having a 0.9 to 1 pound per base box (approximately 0.000055 inch thick) electroplated zinc surface, makes a productparticularly suitable for Venetian blind slats, housings and other accessories. The zinc coating can be applied before the metal is cut to size and shaped, inasmuch as the zinc seems to smearover cut edges and the thin zinc layer is not loosened by fabricating operations. The mixed chromium oxide coating can also be applied before cutting and shaping, and if. desired, even the paint can be coated on the oxide layer before these operations.

An additional feature of the present invention is the cooperation of the mixed chromium oxide coating with a tin underplating, particularly where the tin plating is less than pound per base box. Such aplating covered by a coating of the present invention weighing between about 5 and 10 milligrams per square foot is exceptionally effective in preventing attack of plain carbon steels, such as As the weight of the mixed chromium oxide layer'is reduced below 10 milligrams per square foot, the proportion of reducing agent can usually be diminished, and at a coating weight of about 5 milligrams per square foot, the reducing agent may be eliminated altogether. However, even with minimum coating weights, the use of some reducing agent is advisable to insure the complete insolubility of the heated coating.

The mixed chromium oxide of the present invention will keep the surface of heavy tin layers, those weighing more than pound per base box, brighter longer and is desirable for this purpose. The most effective use of the chromium-containing coating is with tin platings no heavier than about pound per base box. Although any corrodible steel can be so covered, plain carbon types (less than about 2% alloying metals) are least expensive and accordingly most desired. They can have a carbon content varying from extremely low values, 0.05% or even less, to as much as 1.4% or'higher. phoms and sulphur contents can range from substantially zero up to several tenths of a percent. Generally phosphorous maxima are about 0.15% and sulphur maxima about 0.3%. narily considered as carbon steels (SAE 1010 to 1095), free cutting steels, plain carbon tool steels, including those that have up to several ing metals.

The tin plating of the present invention can be applied either electrolytically or by the hot dip process. The lighter platings, however, are much more readily applied electrolytically and electrolytic platings appear to respond better'to the presence of the mixedchromium oxides in the coating directly above them. Any type of electrolytic The phos- These materials include the steels ordia percentof silicon, and cast- 1 pl ting technique can v be usedwith either acid or alkaline,

plating baths. r g

The advantages of the present invention are obtained when the mixed chromium oxide coating is applied over the tin plating, whether or not the tin' plating-is fused. In other words, the final fusing operation normally carried: out-t flow electrolytically deposited tin .plate and thereby change it from a matte to a. shiny surface, can

be omitted, or can be carried out during the final heating operation in which the mixed chromium oxide coating is cured.

It is also desirable, berore applying the chromic acid solution on tin-plated plain carbon steels, to etch the surface of the plated steel with aqueous nitric acid or ferric V nitrate. One to twenty percent solution of either or both of these materials will provide a beneficial etch in from 2 to 20 seconds at a temperature of from 60 to 150 F.

In the following examples the final coated products show a great improvement over untreated or otherwise treated-tin surfaces in resisting pin-point rusting. This can be illustrated by a hot water immersion test, as for example after immersion in water at 200-2l2 F. for

five hours and seventeen hours at 70 F., untreated tinplated steel surfaces show severe pin-point rusting while the same tin-plated steel treated as disclosed in any of the examples, showed-only slight and localized pin-point rusting. v

Example IV I A roll of strip SAElOlO steel 10 mils thick with a pound per base box'ele'ctrolytiotin plating from acid sitannous sulphate bath, was heated at 470 degrees-F. by passing the sheet through a' heating chamber composed of incandescent ceramic cups. The tin plating was fused by this treatment. The fused tinned surface was then flooded with an aqueous solution containing grams per liter of chromic acid and /2 gram per liter of trithanolamine, the solution being held at 80 F. The flooded metal is rolled through resilient rubber rolls wetted with the same solution and then carried between sets of incandescent ceramic gas burners, where it is kept 1 /2 seconds, the metal temperature reaching 300 F.

for about /2 second. The metal is immediately rolled up into coils. The resulting coatingweight is 5 milligrams per square foot.

- 1 Example V Example VI The procedure is the same as ExampleV except 0.75 gram per liter of sucrose is used instead of the .5 gram per liter .triethanolamine and a peak curing temperature of 350 F. is used.

Example VII The procedure is'the same as Example IV except 1.5

grams per liter of triethanolamine are used.

Example VIII Same as Example V except that the tin is electro deposited in an alkaline sodium stannate bath and only sufficient ammonia is added to bring the pH to about 2.5.

. Example IX Same as Example VII except that the pH is adjusted with NH to 5. This coating showed practically the same unusual adhesion as the coating of ExampleIV.

Example X Same as Example .VIII except that the pH is adjusted with NH to 8.5.

' square foot is sufficient,

Example XI Same as Example IV except that the tin is not flow melted after electro-plating and the. chromate treatment is applied to the matte tin surface. 7

Hot "dipped tin coatings are also improved ,by the. treatmentof the present invention, and retain their shininess or luster, much better under humid conditions.

The coatings of the present invention exhibit their outstanding corrosion resistance even without the help of a superficial film of oil. However, such an oil film improves the lubricating of the metal and is advantageously used where the metal is to be handled. Cottonseed oil, palm oil, or any glyceride oil isparticularly desirable since it contributes an unusually large increas in corrosion resistance, and only about 1 milligram per Such a small amount of oil does not interfere with the adhesion of a paint filmapplied directly over it.

The pre-etching with nitric acid, as described above, is suitable for use with hot dip tin coatings, or electroplated coatingsthat are fused or unfused. Such etching will give some improvement when carried out on the tin-plated surface but it will also whiten the tin surface and sharply reduce its luster. It is accordingly preferred to do such etching before the tin plating or before the fusion.

The nitric acid etch path is preferably kept at 70-80" F. or below. Above F. it'generates so much heat, apparentlyby oxidizing some bath ingredients, that its temperature can rise to the point of liberating large quantities of NO and N0 fumes. Acid temperatures lower than 60 F. generally produce etching very slowlyand are accordingly not preferred.

Where preliminary etching is to be carried out on unplated steel used in making food cans or the like, it is even more desirable to monitor the etching operation as by continually inspecting the etched surface. Steel used for other purposes seems to behave about the same even when several thousand milligrams are etched away. per square foot. The etching is preferably just sufiicient to clearly expose the grain boundaries at the metal surface. This usually takes not more than about 400 milligrams per square foot of metal removal. Ordinarily the removal of at least about milligrams per square foot. is needed for best corrosion resistance, but 200 or more milligrams of metal removal'per square foot gives better results with mixed oxide coatings of from 5 to 20 milligrams per square foot, with or without the intervening tin layer.

The continuous inspection of such surfaces can be carried out even while the surfaces are moving at the high speeds of commercial operations, by electronically scanning an optical image of the surface on a television pick-up mosaic with a cathode ray beam, and using the;

resulting electrical impulses to reconstitute an optical duplicate of the image, suitably magnified as desired. In other words, a television camera can be focussed on the surface and used with a television receiver.

The metal surface, even though moving at high speed, can have its image readily scanned by a transversely moving cathode beam that travels fast enough to place successive scanning lines fairly close together and thereby provide the desired degree of. definition, 30 to 200 lines per inch, for example. The forward motion of the image can, in fact, be used to supply the entire spacing between the scanning lines so that the transverse scanning need not have any forward component whatsoever. Electrical pick-up signals can then be provided with scanning pulses directed longitudinally with respect to the image motion so that the successive transverse scanning lines will appear as a side-by-side sequence having a pre-- determined number of lines and then will return for screen having a relatively long persistence, will-appear stationary and can be conveniently examined-for the appearance of the etched surface.

Suitable organic layers for use in the present invention are those usually loosely referred to in the art as enamels,

sanitary enamels or lacquers such as the-'oleo-resinous phenolic or vinyl resinvarnishes. Particularly effective. forms of such organic type coatings are described in US. Patents Nos. 2,231,407, 2,299,433, 2,479,409 and 2,675,334. Such top coatings will even further reduce corrosion as well as increase the wear resistance and lower the contamination of the can contents by the oxides.

A The mixed chromium oxide coating of the present in-: vention can also be used on aluminum surfaces to form coatings that weigh from to 200 milligrams per, square foot. Such coatings, formed in situ as in Examples I to X inclusive, will provide greatly improved adherence for enamel, lacquer, paint or other organic layers, and will also sharply reduce the weathering of the aluminum and the resulting corrosion.

The aluminum surface that is coated according, to the present invention can be either pure iluminum or so-called aluminum base alloys containing over 50%aluminum by weight. For example, the following alloys are very satisfactorily coated: i

Alloying Izggredisnfisfin percnt Alloy Designations y welg t The mixed chromium oxide coatings of the present invention can be applied in weights as high as 200 milligrams per square foot or higher, although little added benefit is derived from the higher weights.

The present invention is also applicable for improving the corrosion resistance and paint adhesion of aluminumcoated metals such as steel that is coated with aluminum by hot dipping, hot rolling, or electroplating. It is also very effective for aluminum foil which, when, coated, makes a much better wrapping material.

The elfectiveness of the coatings of the present invention appears to be greatly increased by the application of polymethylmethacrylate resin layers over the coatings. For example, a 25 to 50 milligram per square foot covering layer of polymethylmethacrylate applied from a solution of the resin in p-xylene strikingly improves the corrosion resistance of SAE 1010 steel carrying a 20 milligram per square foot coating of mixed chromium oxides in accordance with the present invention. This improvement is particularly significant where the steel is subsequently deformed, as in the making of container covers or joints. Wax type resins also show unusual cooperation with the coatings of the present invention when used in the same manner as the polymethylmethacrylate resins. Ordinary waxes and other resins are not as effective and are not even as desirable as palm oil. These resin coatings can have up to 20% plasticizer and/or pigment, and are not driven off in baking operations such as used for curing lithographic plates.

The'mixed chromium oxide coating of the present invention is generally a very pale green in color with a brownish cast when the hexavalent chromium content is on the high side. However, the coating film can be colored by the addition of dyes which should be introduced into the coating film. before the curing, and the dyes should be stable to the curing conditions.

- ment such-as that shown in application Serial No. 433,-

698, filed June 1, 1954. The container of Fig, 2 can be made with soldered joints, for example.

s The pound per base box values given above are standard metal coating terms defined as the number of pounds of coating on 1l2'metal sheets each 14 inches by 20 inches in size.

In some cases, where metals are difiicult to wet, the amount of wetting agent used may lead to foaming. This difficulty may be minimized by adding to the bath an anti-foaming agent such as water solubilized alkyl phosphate type wetting agents. For example, a 0.05% addition of monolauryl phosphate completely esterified with octa'ethylene glycol to a bath' containing 0.3% of the wetting agent of Example I, gives very effective wetting.

The wetting problems are emphasized when the coating bath is applied by electrostatic spraying. This type of spraying is desirably carried out by whirling a film of the bath from the outer edge of a rapidly revolving cup positively charged to 90,000 volts with respect to the surface being coated. ,1 v

The coating can be directed so as to produce a uniform pattern by appropriately located positively charged electrodes. The bath particles are preferably whirled out at an acute angle with respect to the movement of the metal surface in the spray zone, the whirling being directed "so as to follow the moving metal surface.

The mixed chromium oxide coatings appear to conduct electricity to some degree. When used on conductor wires they accordingly increase the current-carrying ability somewhat.

Chemical etching of aluminum for the purpose of improving the adhesion of paints or the like over the coating of the present invention, is not as satisfactory if effected by etching baths such as mixtures of HF, HCl and HNO that leave the metal surface smooth. Any alkali, even sodium carbonate, that can raise the pH of water to about 10, will give good etching although a will give a much more rapid attack. 1

pH of 11 or higher When the metal being coated in accordance with the like that have to be coiled up for convenience in subsequent handling, the curing treatment ispreferably followed by a rapid quench. This keeps the metal going into the coil at a sufficiently low temperature that subsequent thermal changes will not cause any trouble. quench can be very simply carried out by merely passing the metal, as it comes from the curing treatment, over the outer surface of a water-cooled roller. The temperature of the metal before coiling should preferably not be higher than F. v

The mixed chromium oxide coatings of the present invention are very good for reducing metal corrosion caused by chemical influences in addition to those of ordinary weathering or handling. Chromium type stainless steels are, for example, subject to pitting and corrosion when covered with hydraulic cements or plaster of Paris, and this pitting and corrosion is practically completely prevented by the coating of the present invention. This improvement is demonstrated very forcibly with type 430 stainless steel, for instance.

The coatings of the present invention also help to re- ,duce scaling caused by heat treatment of steel, particularly when the coating of the present invention is covered by a top fused layer of inorganic protective material. Type 316 stainless steel will, by way of example, when carrying a 30 milligram per square foot coating of the present invention over which a uniform layer of borax is melted,

Such a heat; treatment for one hour.

scaling whatsoever-when subjected to a2 ,000 F.

practiced otherwise than as specifically described.

. What is claimed is: p

' 1. A method for treating a non-ferrous metal surface comprising the steps of coating the surface with an aqueous solution consisting essentially of water, chromic acid and a compatible reducing agent for the chromic acid, said reducing agent being compatible with the chromic acid for at leastone day at 80 F., and heating the coated surface to atemperature of from 250 to 500 degrees 'F. to cause the reducing agent to react with the chromic acid and leave a Water-insoluble layer, the proportions of reducing agent and chromic acid being such that. from about I 40 to 95;% of the chromium in the chromic acid is reduced to trivalent form, and the final layer Weighs at least about 3 milligrams per square foot of surface.

2. The process of claim .1 in which at least. about /3 of the chromium is reduced and the reducing agent is one that does not leave water-soluble salts in the final. layer.

3; The product produced by the process of claim2.

, .4. The product of claim 3 in which the metal surface is tin plated plain carbon steel in which the tin plating Weighs less than pound per base box, and the reduced chromium oxide layerwveighs no more than milligrams per square foot.

5. The process of claim 1 in which the metal is zinc;

6. The process of claim 1 in which the metal isaluminum.

7. The product produced by the process of claim 5. 8-: The product produced by the processl-of claim5 in which the metal is galvanized steel. 9. The process of claim 1' in which the metal is-copper. 10. A method for treating a surface of aluminum comprising the steps ofcoating the surface with an aqueous solution consisting essentially of water, chromic acid and acompatible reducing agent for the chromic acid, saidreducing agent-being selected from the class consisting of polyalcohols and their oxidation products in which a carbon atom is linked to an alcohol group and to a hydrogen atom, and heating the coated surface to atemperature of from 250 to 500 F. to cause the reducing 1954, and now agent to react with the chromic acid and leave a water insoluble layer, the proportions of reducing agent and chromic acid being such that from about 40 to 95% of the chromium in the chromic acid is reduced to trivalent form and the final layer Weighs at least about 10 milligrams per square foot of surface.

11. The product produced by the process of claim 10.

12. A method for treating a surface of galvanized steel comprising the steps of coating the galvanized surface with'an aqueous solution consisting essentially of water, chromic acid and a compatible reducing agent for the chromic acid, said reducing agent being selected from the class consisting of polyalcohols and their oxidation products in which a carbon atom is linked to an alcohol group and to a hydrogen atom, and heating the coated surface to a temperature of from 250 to 500 F. to cause the reducing agent to react with the chromic acid and leave a water-insoluble layer, the proportions of reducing agent and chromic acid being such that from about 40 to 95% of the chromium in the chromic acid is reduced to trivalent form and the final layer weighs at least about 10 milligrams per square foot of surface.

13. A method for treating tin-plated steel in which the tin plating weighs less than pound per base box, comprising the steps of coating the tin-plated surface with an aqueous solution consisting essentially of chromic acid, ammonia and a compatible reducing agent for the chromic acid, said reducing agent being compatible with the chromic acid for at least one day at F., and heat ing the coated surface to a temperature of from 250 to 500 F. to cause the reducing agent to react with the chromic acid, and to cause the coating to be converted to a water-insoluble layer, the proportion of ammonia inthe solution being such as to provide the solution with a pH between 3 and 10, and the proportions of there ducing agent and chromic acid being such that from about 40 to of the chromium in the chromic acid is reduced to trivalent form, and the final layer Weighs between 3 and 10 milligrams per square foot.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD FOR TREATING A NON-FERROUS METAL SURFACE COMPRISES THE STEPS OF COATING THE SURFACE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF WATER,CHROMIC ACID AND A COMPATIBLE REDUCING AGENT FOR THE CHROMIC ACID, SAID REDUCING AGENT BEING COMPATIBLE WITH THE CHROMIC ACID FOR AT LEAST ONE DAY AT 80* F., AND HEATING THE COATED SURFACE TO A TEMPERATURE OF FROM 250 TO 500 DEGREES F. TO 